The present disclosure relates in general to methods and compositions for producing molded polyolefin and elastomer foams, and more particularly, to foamable particles that may be used, for example, to produce in-situ crosslinked foam cores that can conform to a variety of mold shapes for automotive interior products, marine products, or other molded foam components, and that can furthermore possess heterogeneous chemical and physical properties within the same molded product.
The automotive instrument panel (IP) market may generally be divided into three main categories: 1) Hard IP, 2) Polyolefin (PO) vacuum-formed soft IP, and 3) Polyurethane (PU) foam-in-place soft IP. Hard IP involves injection molding a solid plastic resin into a mold.
PO vacuum-formed soft IP is made by first laminating a sheet of foam with a sheet of thermoplastic olefin (TPO) or polyvinyl chloride (PVC) foil. The combined laminate is heated to ˜170° C., and then placed into a mold with a vacuum to pull the malleable laminate into the mold and make it take the shape of the mold. The substrate may or may not be in the mold at the time of vacuum molding. PO vacuum formed soft IP has benefits such as having low density (e.g., around 4 pcf) and low toxicity, and being lightweight and recyclable.
A shortfall for PO vacuum-formed IP is in limitation to design. Since the foil plus foam bilaminate is heated and vacuum formed to the shape of the mold, the bilaminate is stretched to shape. As a result, wherever there is a deep cavity, the material has to stretch more, causing thinning out of the material, or even a tear in extreme cases. As such, PO vacuum-formed soft IP designs usually do not exceed a height/diameter (H/D) ratio of 0.5 in order to avoid extreme stretching.
PU foam-in-place soft IP is done by placing a preformed TPO or PVC foil on the bottom half of a clam mold, and a substrate under the top clam mold. The molds are closed shut, and then the liquid PU foam precursor is injected into the cavity created by the gap between the top and the bottom molds. The precursor foams inside the cavity and takes the shape of the part, adhering to the foil and the substrate.
A benefit of PU foam-in-place soft IP is the freedom of design. Because the foam precursor is injected into the cavity created by the skin and the substrate, it evenly fills the cavity, providing even haptics and maintaining precise gauge control throughout the part without any concern of tearing due to extreme part design.
However, the disadvantages of PU foam in-place soft IP include but are not limited to its high density (e.g., around 10 pcf), its heavy weight, inability to be recycled, as well various health and environmental hazards. PU's main ingredient is isocyanate, and being exposed to it can cause irritation of the skin and mucous membranes, chest tightness, asthma and other lung problems, as well as irritation of the eyes, nose, throat, and skin. Volatile organic compounds (VOCs) released from the foamed part can cause the same adverse effect on humans especially when the chemicals are not mixed well or remain partially unreacted.
Foamable particles containing a chemical crosslinking agent are described in U.S. Patent Pub. No. 20070249743A1, disclosing a melt-blended composition that can be extruded and cut into pellets or otherwise formed into particles which can be poured or placed into a cavity and expanded. However, chemical crosslinking produces undesirable odors in the foamed product, and does not provide for stable reproducibility of product densities, because the crosslinking level depends on many variables including temperature, time and rate of heating, and in turn, the crosslinking level affects the expandability of the foam. Too high of a crosslinking degree, for example, will result in a rigid foam and inhibit expansion, resulting in higher density than desired.
Furthermore, traditionally a mold cavity is filled with only one kind of foamable material to yield a foam product having a homogenous composition as well as uniform physical properties throughout the product. However, some applications require or would benefit from the use of a foam product having a heterogeneous composition and properties. To impart additional properties to the foam typically requires an inefficient secondary process or multiple manufacturing steps, such as laminating a second foam having the additional property to the first foam to yield a final product having heterogeneous properties such as differing colors, densities, haptics, tensile and elongation strengths, surface characteristics, hardness, etc, imparted by each foam. Such multi-step manufacturing techniques are inefficient and produce foam products having limited geometric and compositional configurations because it is not always possible to combine foams having potentially contradicting chemical and physical properties using known techniques. Furthermore, structural anomalies can sometimes be introduced at the interface of laminated foam products which may lead to product performance inconsistencies or failures, and which can be exacerbated in foam products such as thick foam board formed from multiple laminated sheets.